1. Field of the Invention
The present invention relates to an image forming apparatus configured to transfer a toner image formed on an image bearing member onto a sheet.
2. Description of the Related Art
Conventionally, there has been known an image forming apparatus that transfers a toner image formed on an image bearing member, such as a photosensitive drum, through exposure and development, onto an intermediate transfer member, such as an intermediate transfer belt, and then transfers the toner image formed on the intermediate transfer member onto a sheet. In a full-color image forming apparatus, for example, toner images of respective color components formed on image bearing members, respectively, are transferred in a superimposed manner onto an intermediate transfer member at a first transfer position, and then the full-color toner image on the intermediate transfer member is transferred onto a sheet at a second transfer position.
In order that the image forming apparatus configured as above can form a high-quality image, it is required to transfer a toner image (image) onto a predetermined position on a sheet. For this reason, it is demanded that timing at which a conveyed sheet reaches a transfer position and timing at which a toner image on the intermediate transfer member reaches the second transfer position accurately coincide with each other.
However, variation in the accuracy of stacking of sheets and deformation or aging of mechanism components arranged in a conveying path along which sheets are conveyed can cause a difference between the above-mentioned timings. If an image cannot be accurately transferred onto a predetermined position on a sheet due to this difference between the timings, the quality of the image is degraded.
To solve this problem, there has been proposed an image forming apparatus which is capable of adjusting the timing at which a conveyed sheet reaches a transfer position (see Japanese Patent Laid-Open Publication No. 2004-37916). In this image forming apparatus, the timing at which a toner image on an intermediate transfer belt reaches the transfer position is adjusted by controlling the conveying speed of the intermediate transfer belt.
However, the configuration disclosed in Japanese Patent Laid-Open Publication No. 2004-37916 can cause other kinds of problems concerning image quality (image expansion or contraction and color misregistration).
First, since the conveying speed of the intermediate transfer member, such as an intermediate transfer belt, is variable, a toner image transferred from an image bearing member onto the intermediate transfer member can be expanded or contracted.
Further, in an image forming apparatus which has a plurality of image bearing members associated with respective colors, such as Y (yellow), M (magenta), C (cyan), and K (black), when the conveying speed of the intermediate transfer member changes during transfer of toner images on the respective image bearing members onto the intermediate transfer member, displacement occurs between the respective positions of color-component toner images on the intermediate transfer member. Consequently, there is a fear that the color hue of an image formed on a sheet cannot be obtained as desired.